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II. The Formation of Sulfides

II. The Formation of Sulfides

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268



C. BLOOMFIELD AND J.



K. COULTER



contain large amounts of sulfides, and oxidation of these introduces serious

problems in some lignite, bituminous, anthracite, pyrite, copper, zinc, and

lead mines (Temple and Koehler, 1954). Oxidation of sulfides may provide sulfates for charging ground water in lower-lying areas and these, suggests Poelman (1973b), are the source of sulfur for further pyrite formation in these waterlogged areas. Subsequent drainage and oxidation Iead

to sulfate formation and the creation of “cat sands,” sandy soils with

jarosite mottles. Inland swamps at 2000 m have been described by Chenery

(1953, 1954) in Uganda where the sulfur comes from surrounding formations. Sulfides weather to sulfates and encrustations of sodium sulfate appear in the nearby area, providing salt licks and soluble salts that enter

the swamps in drainage water. Sulfides formed by reduction accumulate

in these swamps. A similar situation has been described by Thompson

(1972) in Rhodesia where the sulfates are thought to originate from deep

seated springs and are then reduced in the peaty swamps in some areas.

Solfataras may also provide excess sulfur in surrounding soils leading to

high acidity.

Sulfur from biological materials, algae, diatoms, etc., is described by

Sombatpanit (1970) as the source of sulfides in some “gyttja” soils in

Sweden. These soils are formed by the simultaneous sedimentation of fine

mineral particles and plant and animal remains in rivers and lakes. Other

gyttja deposits in Sweden and Finland originate in marine or brackish

environments.



B. SULFATE REDUCTION

IN ANAEROBIC

SOILS

When Hamlet asked, “How long will a man lie i’ th’ earth ere he rot?’

he was told, “. . . some eight year. A tanner will last you nine year.”

Shakespeare may have underestimated the effect of tannins on protein, as

the preservation of Iron Age bodies for 2000 years in Danish peat bogs

seems to have resulted from some such process (Glob, 1971). The stomach contents of these bodies have been preserved well enough to permit

identification of the various grains that constituted the last meals, so that

digestive processes must have ceased quite soon after death. It seems unlikely that tannins would diffuse rapidly enough to account for this, and

it has been suggested that the preservation of the stomach contents results

from the action of hydrogen sulfide.



I . Sulfur Reducing Organism

Hydrogen sulfide is formed in peat bogs, etc., as a product of putrefaction. Bacteria of the genus Clustridium are chiefly responsible for the

anaerobic decomposition of protein, but another group of bacteria is a



GENESIS AND MANAGEMENT OF ACID SULFATE SOILS



269



much more important source of hydrogen sulfide in anaerobic soils. These

are the dissimilatory sulfate-reducing bacteria, an exclusive property of

which is the utilization of sulfate in the same sense that higher organisms

use oxygen-i.e., sulfate acts as the terminal electron acceptor for their

respiratory processes. Postgate ( 1959 ) suggested the term assimilatory sulfate reduction to describe the production of sulfur-containing organic cell

constituents, and dissimilatory sulfate reduction for the relatively much

more extensive energy-yielding process. The process can be formalized as:

2CHa.CHOH.COOH



+ SO



1-



+ OCH3.COOH



+ 2C02 + 2H20 + S*-



Stoichiometric amounts of sulfide are formed, and if adequate sulfate is

available, as in marine or estuarine environments, much hydrogen sulfide

is formed at the expense of relatively little organic matter-as much sulfide

would be formed by sulfate-reducing bacteria from 1 mole of lactic acid

as would be provided by several kilograms of putrefying protein.

Beijerinck described the original type species, Desulfovibrio desulfuricans, in 1895. Campbell and Postgate (1965) distinguished two genera,

sporulating Desulfomaculum, and Desulfovibrio, which does not form

spores (for recent reviews, see Postgate, 19.60, 1968; Roy and Trudinger,

1970; Kelly, 1970). Both occur in soils and waters. They are obligate

anaerobes, but they are not killed by exposure to air, and although they

do not become active unless the environment becomes anaerobic, they

seem to occur in almost all damp terrestrial environments. Desulfovibrio

also reduces thiosulfate, tetrathionate, and colloidal sulfur to sulfide (Postgate, 1959).

In biological sulfate reduction experiments, in which the redox potential

was controlled automatically, Connell and Patrick (1968) found that sulfate became unstable at about -150 mV. Under their conditions, the bacterial reduction of sulfate was confined to the pH range 6.5-8.5, which

is a less acid lower limit than usual; pH 5 seems to be about the lowest

value at which anaerobic sulfate reducers are active (Bloomfield,

unpublished).

Hydrogen sulfide diffuses readily, and, unless it is immobilized as an insoluble sulfide, it tends to enlarge the anaerobic zone and extend the environment favorable to the development of sulfate-reducing bacteria.

Sulfate reducers have important effects in causing the precipitation of

metal sulfides, notably of iron, in causing pollution of waters, etc., and

they are responsible for the corrosion of steel buried in certain anaerobic

soils.

The reduction of elemental sulfur to sulfide is a widespread reaction

among bacteria, yeasts, fungi, etc. (Roy and Trudinger, 1970). Bromfield

( 1953) found that, after partial sterilization with carbon tetrachloride,



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C. BLOOMFIELD AND J. K. COULTER



some soils evolve hydrogen sulfide when incubated aerobically with sucrose

and ammonium sulfate. Partial sterilization apparently killed bacteria that

inhibit the formation of hydrogen sulfide by Bacillus megatherium, which

was identified as the responsible organism.

2. Sulfide Oxidizing Organisms



Certain lithotropic organisms, which use either radiant energy or the

energy released by the oxidation of inorganic compounds for their synthetic processes, oxidize reduced sulfur compounds (Postgate, 1968; Kelly,

1971 ) . One group, the colorless sulfide-oxidizing bacteria, take advantage

of a chemically unstable system in which dissolved air and hydrogen sulfide

coexist. They occur at the interface of aerated and hydrogen sulfide-containing water, both fresh and saline. The most common members of this

group are the filamentous sulfur bacteria, which often occur as a white

scum in polluted water. Beggiatoa seems to be the member of this group

about which most is known. It occurs in fresh and marine waters; it was

originally thought to be strictly autotrophic, but heterotrophic strains have

been isolated. Sulfur granules are deposited within the cell during the oxidation of sulfide, and sulfate appears later in the medium. Truper and

Hathaway (1967) found that sulfur formed by strains of purple and green

bacteria (Thiocystis violaces, Chromutium, and Chlorobium ) gave diffraction lines agreeing with those of orthorhombic sulfur.

Photolithotropic bacteria accomplish the oxidation of sulfide under

anaerobic conditions by linking the process to the photoreduction of carbon dioxide. Members of this group are colored green (Chlorobium) or

purple (Chromatium), according to the relative contents of chlorophyll

and carotene. Both form elemental sulfur during the oxidation of sulfide.

They closely resemble the blue-green algae, some of which are also capable

of oxidizing sulfide. They often occur in gelatinous masses.

The various reducing and oxidizing sulfur bacteria together can form

an ecosystem-the sulfuretum (Postgate, 1968). Hydrogen sulfide is generated in the anaerobic lower levels, and nearer the surface, at depths to

which light can penetrate, autotrophic bacteria, including colored sulfide

oxidizers, fix carbon dioxide and nitrogen, and oxidize sulfide. Higher still,

at the fringe of the anaerobic zone, colorless sulfide oxidizers produce elemental sulfur which is oxidized to sulfate by Thiobacilli, which are discussed later. The cycle can continue indefinitely provided minor elements,

light, air, and sulfate remain available. Sulfureta occur widely; the Dead

Sea is a notable example, but they need be no bigger than a few grains

of sand.

3. Development of Alkalinity

The microbial reduction of sulfate in anaerobic soils is accompanied

by the formation of carbon dioxide; the net result of this and the hydrolysis



GENESIS AND MANAGEMENT OF ACID SULFATE SOILS



27 1



of soluble sulfides is the formation of bicarbonate, and increased alkalinity.

Verner and Orlovsky ( 1948) detected sulfate-reducing bacteria in saline

soils, particularly in peaty and bog solonchaks where anaerobic conditions

prevailed; they suggested that sulfate-reducing bacteria are responsible for

the development of solonchaks and the accumulation of soda. Abd-ElMalek and Rizk (1963) concluded that microbial sulfate reduction was

mainly responsible for the formation of natron, i.e., hydrated sodium carbonate, in Wadi Natrun in the Libyan desert. Janitzky and Whittig (1964)

found an equivalent relationship between the amounts of sulfate reduced

and bicarbonate formed. Ogata and Bower (1965) observed no appreciable reduction of sulfates in flooded and-zone soils, unless the organic

matter content exceeded 5 % , or undecomposed plant residues were

present. Szabolcs (1966) considered biological sulfate reduction to be responsible for the formation of alkali (szik) soils in Transdanubia. Hardan

(1973) showed that the accumulation of carbonate in soils of the

Mesopotamian plain results from microbial sulfate reduction.

Thus acid sulfate soils could not form in a closed system in which the

bicarbonates were preserved; mostly these are lost by leaching at the time

of sulfide formation.

OF IRON SULFIDES

C. FORMATION



Sulfide formed in sediments tends to be precipitated by heavy metals,

and such deposits are usually stained black by ferrous sulfide. Siebenthal

(1915, quoted by Roy and Trudinger, 1970) considered biological sulfate

reduction to have been involved in the formation of some zinc sulfide deposits in the United States; it seems to be at least a theoretical possibility

that sulfate-reducing bacteria were responsible for the formation of some

base metal sulfide deposits (ZoBell, 1946, see Postgate, 1960; Miller, 1950;

Baas Becking and Moore, 1961; Dunham, 1961). The bacterial formation

of iron sulfides is the process responsible for the accumulation of inorganic

sulfides in soils and sediments, and is thus the ultimate cause of B e formation of acid sulfate soils.

The conditions under which iron sulfides are formed has been intensively

studied by Berner (1962, 1964, 1967, 1970, 1972), Roberts et al. (19691,

and Rickard (1969a,b, 1973). According to Rickard (1973) seven iron

sulfides are known: pyrrhotite; Fe,l_,,S, x = 0-0:126, hexagonal or

monoclinic; mackinawite, c. FeS, tetragonal; cubic ferrous sulfide, c. FeS;

griegite, Fe,S,, cubic; smythite, Fe,S,, hexagonal, possibly monoclinic;

pyrite, FeS,, cubic; marcasite, FeS, orthorhombic. Rickard ( 1969a) prepared all but smythite and cubic ferrous sulfide in bacterial sulfate reduction experiments.



272



C. BLOOMFIELD AND J. K. COULTER



In relation to the formation of acid sulfate soils, the essential aspect

of sulfide formation is the accumulation of iron disulfide, which usually

occurs as pyrite, but occasionally as marcasite.

1 . Formation of Pyrite and Marcasite

Allen et al. (1912) prepared iron disulfides by heating sulfur and ferrous sulfide in sealed tubes. The formation of marcasite was favored by

acid conditions, whereas pyrite was formed near neutrality. Pyrite and marcasite were distinguished by chemical and optical crystallographic methods.

Verhoop (1940) prepared pyrite by anaerobic incubation of ferrous sulfide

and elementary sulfur at room temperature. Berner (1962) investigated the

effect of hydrogen sulfide on various iron compounds, and concluded that

the disulfides are formed only in the presence of elementary sulfur, which

in his experiments was produced by the oxidation of hydrogen sulfide by

either ferric iron derived from geothite, or atmospheric oxygen. Pyrite was

formed only below pH 5, and marcasite below pH 3.5. Rickard (1969b),

again in inorganic systems, obtained pyrite and marcasite over the pH

range 4.4-9.5; the proportion of marcasite, which predominated at pH 4.4,

decreased to zero as the pH was increased to 9.5. The effect of pH on

the form of the product is consistent with Rickard’s conclusion (1973)

that whereas marcasite is formed by the oxidation of mackinawite by

elementary sulfur, pyrite results from the interaction of makinawite and

polysulfide, the latter being stable only under relatively alkaline conditions.

It is interesting to note that Senarmont (1851, quoted by Allen et al.,

1912) prepared the disulfide by heating ferrous salts with alkaline

polysulfides.

The reaction of sulfur with ferrous sulfide is quite rapid, being 50%

complete after vigorous stirrings for 1 hour at 4OoC, although in unstirred

bacterial sulfate-reduction experiments sulfidation of geothite was not complete after 3 months (Rickard, 1973).

Kaplan et al. (1963) found elemental sulfur in recent marine sediments,

but befork extraction of the sulfur the samples were treated with dilute acid

to decompose monosulfide, so that if acid-soluble ferric compounds were

present at least some of the sulfur could have been formed by oxidation

of hydrogen sulfide by Fe3+during the preparation of the samples. As elementary sulfur is reduced by D. desulfuricans, the formation of iron disulfide in anaerobic sediments must be the net result of competition between ferrous sulfide and sulfate-reducing bacterias for sulfur, as it is

formed by the reaction of hydrogen sulfide with ferric iron; it is to be expected that elementary sulfur would have only a transient existence in an

anaerobic sediment.



GENESIS AND MANAGEMENT OF ACID SULFATE SOILS



273



D. ENVIRONMENTAL

FACTORS

AND SULFIDE

FORMATION

Essential for the formation of sulfides is a supply of sulfates and organic

matter, so coastal and deltaic areas, often very important agriculturally,

provide optimum conditions for formation of sulfides.

1 . Physiography

Knowledge of the physiological conditions for formation of sulfide-bearing muds is useful for understanding their genesis, predicting their location

and mapping their boundaries. Recent coastal deposits cover very large

areas, particularly in the tropics; an example is the west coast of Malaysia

where the deposits may be up to 40 miles wide and 450 feet deep (Carter,

1959). Deltas form at the mouths of all rivers, but those in the tropics

originate from much larger rivers, are flooded to much greater depth at

certain times, and are desiccated more intensely at others.

Fosberg ( 1964) has classified the main physiographic features of deltas

into water (distributaries, delta channels and tidal channels, lakes and

ponds of levee bank depressions, and abandoned channels), wet lands

(filled lakes and ponds), and drylands (delta terraces, natural levees, and

sand ridges and flats). The distributaries tend to be the larger channels

in the delta; they have natural levees stabilized by vegetation, and strong

currents may provide considerable scouring action. Lakes and ponds may

be little subject to active tidal influence so they normally contain fresh

water, often leading to freshwater peat formation. Interconnecting basins

originate as areas of shallow sea that are subsequently cut off by ridges

and sand bars as the delta extends seaward. In the tropics, mangrove grows

along the margins and in many parts of Asia, Nipa palm, Nipa fruclecens

becomes established. Lakes and ponds gradually fill with silt and organic

debris; where the area is subject to tidal inundation, reeds and tall grasses

dominate in the fresher water areas and mangrove in the saline areas. Interconnecting basins, lakes, and ponds, that are subject to tidal influence,

thus provide the optimum environments for sulfide formation, but changes

in sea level can inundate former fresh water peat areas with sea water,

thus leading to sulfide formation in these as well.

Van der Kevie (1972) shows that normal soils are associated with river

levees and, where these are broad the areas of sulfide formation may be

relatively small. Microrelief and sulfide formation are closely related; areas

of slightly higher elevation may never have had any sulfides, or the higher

relief may indicate sulfide-free material deposited over sulfidic muds,

whereas the lower areas have had no such deposits or very thin layers

only (Brinkman and Pons, 1973).



274



C. BLOOMFIELD AND J. K. COULTER



The position of acid sulfate soils in the landscape can also be related

to sea-level changes. Those formed when sea levels were higher may now

be a long distance from the sea and have better drainage and consequently

strongly developed acid sulfate soil characteristics. Old Pleistocene terrace

soils have also been reported as having some acid sulfate soil properties

(Pons and van der Kevie, 1969; Moorman, 196 1 ) .

Thus potential acid and acid sulfate soils may be found in coastal areas

with saline or brackish water influence, in seasonal or permanent freshwater swamps, formerly brackish, in Pleistocene terraces and in high altitude swamps with adjacent sources of sulfate.

Physiographic variations and sea-level changes have thus given a variety

of depositional features so that acid sulfate soils may vary from large contiguous areas to small patches of only a few hectares. Van der Kevie

(1972) recorded that acid sulfate soils in Central Thailand cover an area

of 800,000 ha with only small inclusions of nonacid soils; the Plain de

Joncs in Vietnam also covers a very large area of uniformly acid sulfate

soils. In other areas, e.g., Malaya, Sarawak, the Netherlands, acid sulfate

soils often occur in patches of a few to perhaps several hundred hectares,

interspersed in nonacid soils.

2 . Vegetation

Although much hydrogen sulfide is formed at the expense of relatively

little organic matter (Section 11, B), Rickard (1973) states that organic

matter normally limits sulfate reduction; thus sources of organic matter

and hence the vegetation associated with coastal sediments are of great

importance. Sediments in coastal regions develop over a wide range of tidal

regimes varying from continuously flooded, flooded twice daily at high tide,

twice monthly or only occasionally. Tidal ranges may be very great and

the salinity of the water is obviously much influenced by river flow. Parts

of West Africa receive more than 4000 mm of rain in 6 months, with

virtually none for the rest of the year. There the large estuarine areas are

fresh during the rains and saline during the dry season. Areas flooded only

occasionally by high tides may accumulate considerable quantities of salt,

which render them almost sterile. Such barren flats (tannes) are extensive

in Gambia and Senegal.

Vegetation successions in coastal areas play a major role in the buildup

of sediments by trapping the mud and controlling erosion. Mangrove successions have been particularly well studied because of the value of certain

species for timber, charcoal and tanning materials. A useful summary of

salt marsh vegetation in temperate zones is given by Steers (1959).

Zostera sp. are early colonizers, growing on soft and wet mud, trapping

silt so that banks grow sufficiently high for ill-defined creeks to form. As



GENESIS AND MANAGEMENT OF ACID SULFATE SOILS



275



the creeks are cut off by further siltation, small dams form; these lead

to salt pans which may remain devoid of vegetation until the drainage

system changes and salts leach out. Spartina alterniflora is an important

colonizer on the United States east coast.

a. Mangrove. In contrast to the low-growing herbaceous plants that

colonize salt marshes of temperate areas, tropical salt marshes have several

tree species as primary colonizers, and mangrove, a term used to cover

both the ecological group of species on tidal lands of the tropics and the

plant communities that include these species (Richards, 1952), is primarily

involved. There are several families of mangrove, the more important being

the Rhizophoraceae, the Lythraceae, and the Verbenaceae. Rhizophora

mucronate and R . conjugata occur in the Malaysian area and eastern and

southern Africa (Watson, 1928; Dale, 1939; Macnae and Kalk, 1962).

Rhizophora apiculata and R . stylosa are found in Queensland (Macnae,

1966). Around the Atlantic shores the three main species are Rhizophora

mangle, R . racemosa, and R . harrisonii (Keay, 1953; Davis, 1940). Various species of the genus Avicennia occur in the Verbenaceae family. Avicennia germinans occurs in West Africa, A . oficinalis, A . intermedia, and

A . alba in Malaysia, A . marina in Australia, eastern and southern Africa,

and A . nitida in Florida.

In this discussion our interest lies in the relationships, mostly indirect,

between vegetation type and sulfide levels in the mud. Unfortunately

studies on the colonizing vegetation seldom include any information on

the muds, except perhaps on the salinity, but sulfide contents are seldom

recorded. Reeds (Phragmites sp.) are usually related to sulfides in temperate area muds, but these are not primary colonizers of coastal areas.

This suggests that, in these areas, excess sulfides are formed when the soils

are under intermittent flooding by saline waters, i.e., brackish water

swamps. Work in West Africa (Tomlinson, 1957) appears to have first

drawn attention to the relationships between mangrove species and excess

sulfide formation; he found that areas presently or formerly under

Rhizophora racemosa developed much acidity on drainage, whereas soils

from areas of Avicennia did not. He attributed this to differences in the

rooting habits of the species; stilt roots of one Rhizophora tree may cover

an area of 6 m diameter (Watson, 1928). In the soil these stilt roots are

covered with root hairs, the major source of the peaty material which gradually builds up under this vegetation. This peaty layer may be several feet

thick in West Africa, and Rosevear (1947) states that it can be cut and

burned as fuel. Such thick peat deposits have not been reported from

Malaysia, but in Thailand van der Kevie (1973) states that the vegetation

over broad areas of sulfide muds has been swamp forest, “probably

Rhizophora.” The reasons for the different amounts of fibrous peat formed



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C. BLOOMFIELD AND J. K. COULTER



under Rhizophora in various parts of the world may lie in the reaction

of the tree to ecological factors. Watson (1928) in Malaya, and Davis

(1940) in Florida, recorded that trees growing in shallow soils or areas

subject to very deep inundation have the greatest mass of roots, so that

large root masses would be expected in some areas of West Africa, because

of the great changes in water level that occur under tidal and fresh water

flooding.

By contrast Avicennia sp. produce a shallow widespread root system,

with small pneumatophores protruding from the surface. In West Africa,

soils under this species are not usually fibrous unless previously covered

with Rhizophora. On the other hand, Davis (1940) reports that in Florida

deep peats are formed from the remains of both Rhizophora and Avicennia

sp. Peats formed from Rhizophora remains have an abundance of reddish

brown pithy roots in varying stages of decomposition. The reddish brown

color distinguishes this peat from that formed by Avicennia, which is

darker, has more yellow roots and is more plastic. Vann (1969) described

soils under Avicennia in northeast Brazil that have 15-35 cm of partially

decayed vegetation over a brownish gray clay.

Because of the behavior of the different species of mangrove in peat

formation, and consequently sulfide formation, mangrove successions are

of interest. As they require protection from erosion, mangroves generally

do not colonize exposed coastlines; on such coastlines they flourish only

where there is a wide, shallow sea bed to reduce wave action, as on the

west coast of Malaysia. Development more usually takes place in protected

bays and behind sand bars, and in this well protected environment organic

debris is not swept away, and so can supply the energy for bacterial reduction of sulfates.

Mangrove successions parallel to the coast can be related to the degree

of tidal immersion. Savory (1953) described such zonation in West Africa,

the pioneer genus being Rhizophora, which extends to the limit of the

diurnal tides and is flooded twice daily. R. racemosa is the pioneer species,

and may reach heights of 40 m. R . mangle occurs at the drier inner limits

of the zone as a small shrub up to 4 m high. By contrast Davis (1940)

reported R . mangle in Florida as a large forest of tall trees growing in

almost continuously submerged soil. In both Florida and West Africa the

landward side of the Rhizophora is colonized by Avicennia nitida, on land

that may be regularly or only occasionally flooded with brackish water.

The succession described by Giglioli and King (1966) in Gambia is Rhizophora racemosa followed by Avicennia germinans; as the soils become drier

these die out and barren flats take their place, as the occasional saline

floods leave the land too salty for the mangrove.

In Malaya Watson (1928) recorded a different type of succession, with

Avicennia the primary colonizer. Carter (1959) stated that an accreting



GENESIS AND MANAGEMENT OF ACID SULFATE SOILS



277



coastline is shown by Avicennia advancing seaward as small seedlings, a

stable coastline by mature Avicennia, and an eroding coastline by Rhizophora. The more important factors that determine the colonization pattern

are the erosion conditions and the nature of the mud, Avicennia colonizing

sand and firm soils, Rhizophora soft muds.

Andriesse et al. (1973) showed that in Sarawak the vegetation succession in deltaic areas is Nibong palm (Elncosperma filamentosa) in the

higher areas, where salinity is around 0.5 mho/cm; A vicennia, Rhizophora

near rivers and creeks, where the salinity is highest ( >30 mho/cm) ; and

Nipa palm (Nipa fructicens) over the areas of intermediate elevations and

salinities. Nipa palm appears to play an important role in sulfide formation

in the Malayasia region, for acid sulfate soils very often contain the roots

of this plant.

b. Other Vegetation. While there are some obvious relationships between present day vegetation and sulfide-bearing muds, in many areas the

vegetation of either potential or actual acid sulfate soils is often quite

different from that under which the soils were originally formed. Such areas

may now support a limited range of species of grasses, sedges, shrubs, and

trees. Sedges (Phragmites vulgaris) and Cyperus sp. have been reported

from Gambia. Pure stands of gelam, Melaleuca leucadendron have been reported from Malaya and Indonesia; Fimbristylis globulosa has also been

reported from Indonesia by Driessen (1973), and Brinkman and Pons

( 1973) report several species of grasses, e.g., Imperata brasiliensis, sedges

(Scleria and Rhynochospora sp.), and trees (Tabebuia insignis). Sedges

seem to be the major type of vegetation in large areas of Vietnam (The

Plain de Joncs) . However, these species, although useful indicators, are

not specific for acid sulfate soils, but they are indicative of generally

adverse soil conditions.

As a conclusion, certain vegetation consociations are indicative of potential or acid sulfate soils, but none are specific; the closeness of the relationships obviously varies from one environment to another, so that extrapolation of relationships that exist, say in West Africa, to East Asia is

unreliable.

3 . Climate



Acid sulfate soils occur in a wide variety of climates, but the largest

areas are in the humid and monsoonal zones of the tropics, and in the

moist temperate climates. The temperature obviously influences the

amount and type of vegetation; although more organic matter is present

in sediments of temperate areas at the time of deposition, the continuous

growth of vegetation in the tropics can add larger quantities to the deposits.

Rainfall distribution affects the behavior of sulfidic muds after deposition; without a marked dry season they may remain in a waterlogged and



278



C. BLOOMFIELD AND J. K. COULTER



reduced state, unless drained artificially. In monsoon areas the soils dry

out to a meter or more during the dry season, and strongly acid conditions

can thus develop. Even in the wet tropics, prolonged dry spells, though

infrequent, do occur, and Dunn (1965) reported the mass death of fish

and the flocculation of sediments in a river in West Malaysia following

a prolonged dry spell that caused swamp areas to dry out; subsequent heavy

rain washed the sulfates into the river. The strongly contrasting climatic

conditions are of great importance in management, and are discussed in

greater detail in Section VII.

In temperate areas the soils are usually so swampy that they seldom

dry out naturally.

4. Fauna

The influence of soil fauna on acid sulfate soil formation has not been

widely studied, but Andriesse et al. (1973) have contributed some interesting observations on the activities of the mud lobster (Thallusinu anornula)

in mangrove and Nipa palm swamps in Sarawak, where the lobsters channel

to a depth of 120 cm or so and build mounds as high as 150 cm, covering

as much as 40% of the land surface. Apparently the lobsters consume

much of the organic debris from Nipa palms, etc., and thus have much

the same effect as earthworms in dry soils. As a consequence of these deep

burrowing activities, sulfidic materials are brought up from the deep subsoils, and these then develop extreme acidity (pH 2.7-3.9). Leveling the

lobster mounds for agricultural development thus spreads a layer of highly

acid material over the surface. Micromorphological studies by Slager et ul.

(1970) on soils from Surinam and Thailand showed that crabs were active

in homogenizing some of the upper soil horizons.

Macnae and Kalk (1962) gave details of the soil fauna found in mangrove areas of Mozambique and described the wide variety of crabs and

small fish and oysters on Rhizophoru roots, but they did not report any

mound-building lobsters. Such animals appear to be absent too from any

other extensive area of sea littoral tropical vegetation, but they obviously

have considerable significance in the pattern of soil development.

Ill.



A.



Oxidation of Sulfides



MICROBIAL

REACTIONS



The acidification of mine waste has been the subject of intensive research, and our present knowledge of the oxidation of pyrite under natural

conditions derives largely from the efforts of workers in this field (Temple

and Koehler, 1954; Lorenz, 1962; Clark, 1966; May and Berg, 1966;



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